a novel high-power pulse pecvd method
TRANSCRIPT
Surface & Coatings Technology 206 (2012) 4562–4566
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Surface & Coatings Technology
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A novel high-power pulse PECVD method
Henrik Pedersen a,⁎, Petter Larsson a, Asim Aijaz a, Jens Jensen a, Daniel Lundin a,b
a Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Swedenb Laboratoire de Physique des Gaz et Plasmas, UMR 8578 CNRS, Université Paris Sud-XI, F-91405 Orsay Cedex, France
⁎ Corresponding author. Tel.: +46 13 28 68 47; fax:E-mail address: [email protected] (H. Pedersen).
0257-8972/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.surfcoat.2012.05.007
a b s t r a c t
a r t i c l e i n f oArticle history:Received 20 March 2012Accepted in revised form 4 May 2012Available online 11 May 2012
Keywords:PECVDHollow cathodePulsed plasma dischargesAmorphous carbon
A novel plasma enhanced CVD (PECVD) technique has been developed in order to combine energetic particlebombardment and high plasma densities found in ionized PVD with the advantages from PECVD such as a highdeposition rate and the capability to coat complex and porous surfaces. In this PECVDmethod, an ionized plasmais generated above the substrate by means of a hollow cathode discharge. The hollow cathode is known to gen-erate a highly ionized plasma and the discharge can be sustained in direct current (DC) mode, or in high-powerpulsed (HiPP) mode using short pulses of a few tens of microsecond. The latter option is similar to the powerscheme used in high power impulse magnetron sputtering (HiPIMS), which is known to generate a high degreeof ionization of the sputteredmaterial, and thus providing new and addedmeans for the synthesis of tailor-madethin films. In this work amorphous carbon coatings containing copper, have been deposited using both HiPP andDC operating conditions. Investigations of the bulk plasma using optical emission spectroscopy verify the pres-ence of Ar+, C+ as well as Cu+ when running in pulsed mode. Deposition rates in the range 30 μm/h have beenobtained and the amorphous, copper containing carbon films have a low hydrogen content of 4–5 at%. Further-more, the results presented here suggest that amore efficient PECVD process is obtained by using a superpositionof HiPP and DC mode, compared to using only DC mode at the same average input power.
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
A chemical vapor deposition (CVD) process that uses the energyfrom a plasma discharge, rather than thermal energy supplied by highprocess temperature, to activate the gas phase chemistry is denotedplasma enhanced CVD (PECVD) or alternatively plasma activated CVD(PACVD). In a PECVD process, the precursor gases are cracked by the en-ergetic species (predominantly through electron impact collisions [1])in the plasma at a low overall process temperature, which means thatlow substrate temperatures can be achieved during deposition. Thisenables deposition on temperature sensitive substrates and the use ofprecursors with low reactivity. A PECVD system is classified as a directplasma process, when the substrate to be coated is placed directly inthe plasma discharge, or a remote plasma process, when the plasma iscreated at some distance from the substrate. The plasma is commonlycreated by applying an electric field across a volume of gas using for ex-ample an RF glow discharge [2,3].
Another approach to ignite a plasma is to use a hollow cathode dis-charge [4,5]. The hollow cathode traps electrons in a hollow cylinder orbetween two parallel plates. It works like two electrostatic mirrorsreflecting electrons between the sheaths until they are thermalizedthrough collisions, and thus increasing the plasmadensity and theprob-ability of ionizing any material passing through [6]. The hollow cathode
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approach has previously been utilized in PECVD processes for the depo-sition of amorphous carbon films and the process has been studied bothby experiments and modeling [7] and up-scaling by the use of an arrayof hollow cathodes has been demonstrated [8]. Coating of the inside ofmetallic tubes with amorphous carbon has been done by using thetube itself as a hollow cathode and introducing the process gases inthe tube [9]. The hollow cathode PECVD method has also been usedfor amorphous CN films [10].
During the last 15–20 years, thefield of ionized physical vapor depo-sition (iPVD) has grown rapidly [11] especially by the introduction ofthe high power impulse magnetron sputtering (HiPIMS) technique,also known as high power pulsed magnetron sputtering (HPPMS),which uses high-power pulses at a low duty factor (b10%) and low fre-quency (b10 kHz) leading to peak cathode power densities of the orderof several kW cm−2, which is known to generate a dense and highlyionized plasma [12].
There have been reports on the usage of the power scheme fromHiPIMS in PECVD for depositing α-Al2O3 [13], using a parallel platePECVD reactor. This study reported an increased hardness of thefilms deposited when using high-power pulses compared to conven-tional PECVD, which was attributed to a better cracking of the AlCl3precursor by the higher plasma density, leading to lower chlorine in-corporation in the films. Furthermore, a pure PVD process (sputtering)using a hollow cathode in combination with HiPIMS has been demon-strated to give high deposition rates of metallic films [14].
By using a highly ionized plasma in a PECVD process, the depositionconditions can be designed to provide a higher degree of energetic
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bombardment of the growing film. The energetic bombardment isachieved by applying a negative substrate bias at the substrate whichwill accelerate the positively charged ions in the plasma towards thesubstrate. Such a bombardment is highly important for deposition ofsp3-hybridized films like cubic boron nitride [15] and diamond likecarbon [16].
In this paper, a novel PECVD thin film deposition process, based ona hollow cathode discharge plasma and a power scheme using high-power pulses, is presented and applied to deposit copper containingamorphous carbon films at an exceptionally high deposition rate.Thin films of amorphous carbon (a-C) are among the most promisingmulti-functional material systems for a number of industrial applica-tions today, e.g. hard coatings, biomedical coatings andMEMS devices[17]. Furthermore, copper containing a-C films, with copper contentas high as 77 at%, have previously been studied and promising tribo-logical and anti-bacterial properties have been reported [18–20]. Thegrown thin films have been characterized in order to investigate theelemental composition, microstructure as well as hardness. The re-sults are also correlated to the plasma conditions as studied by opticalemission spectroscopy.
2. Experimental details
The novel PECVD reactor developed in this study (Fig. 1) consisted ofa reaction chamber made of stainless steel, which was pumped by aturbo molecular pump to a background pressure of less than 0.1 mTorr.The hollow cathode was placed in the top lid of the reaction chamberover the substrate. A hollow cathode made of copper was chosen dueto the high electrical conductivity, but in principal any conductingmaterial could be used. The diameter and length of the hollow cathodewere 5 and 85 mm, respectively, which gives an inner cathode area of1340 mm2. Argon gas (minimum purity of 99.9997%) was fed throughthe hollow cathode enabling a brightly glowing argon plasma jet exten-ding approximately 10 mm from the hollow cathode towards the sub-strate. The argon flow is controlled by a mass flow controller set at84 sccm throughout the entire study. For the carbon films deposited inthis study, acetylene (C2H2) (99.6%) was used as precursor. The precur-sor gas flow was controlled by a mass flow controller and was fed fromthe side into the plasma jet. The total process pressure was adjusted bya throttle valve. In this work the pressure was varied between 450 and550 mTorr. The cathode-substrate distance can be modified by movingthe sample holder. In this work a cathode-substrate distance of 20 mmwas used.
To ignite the hollow cathode plasma, either DC power or a combina-tion (superposition) of DC and high-power pulses (HiPP) was used by
Fig. 1. Schematics of the hollow cathode PECVD reactor.
connecting one DC power supply (MDX 500, Advanced Energy) andone high-power pulsed power supply (HiP3, Ionautics) in parallel. Inthe HiPP+DC mode the DC power supply was isolated from the HiPPpower supply by a series diode between DC supply and cathode. Forpure DC processes, only the DC power supplywas connected to the hol-low cathode. The discharge parameters, such as cathode voltage, UD,and cathode current, ID, was monitored and recorded on a TektronixTDS2004B oscilloscope. PECVD processes using a total applied averagepower of 50–200W in DC or HiPP+DCmodewere studied. The reasonfor using a superposition of HiPP+DC and not only HiPP is that it facil-itates the ignition of the plasma and thus leading tomore stable operat-ing conditions. Pulses of approximately −450 V and 12 A were usedwith a pulse duration of 30 μs and a repetition frequency of 500 Hz(see Fig. 2). These discharge conditions resulted in an average HiPPpower of 30W to the cathode and the total input power was variedby adjusting the DC power.
A spectrometer (Mechelle Sensicam 900) connected to a collimatorvia an optical fiber was used to record the emission from the plasma.The spectral range of the spectrometer was 300–1100 nm. Time-averaged optical emission was recorded from a volume of plasmalocated between the cathode and the substrate.
Single crystalline (100) silicon wafers, cut into approximately2×2 cm2 pieces, were used as substrates. The substrates were ultra-sonically cleaned in ethanol for 10 minutes and blow dried in dry ni-trogen before loading them into the deposition chamber. Prior todeposition the substrates were plasma etched in a pure argon atmo-sphere at 45 mTorr, by applying a large negative pulsed bias with afrequency of ~100 kHz to the substrate holder, giving a self-bias volt-age of about −600 V. During deposition the ions in the plasma weredirected towards the substrate surface using the same negativepulsed bias. In this case the self-bias voltage was varied in the range−16 V to −200 V, leading to an intense ion bombardment duringfilm growth. The plasma was further guided towards the substrateby the position of the anode placed below the substrate holder; ananode bias of +40 V DC was used throughout this study.
The thickness andmicrostructure of the films were studied using aLEO scanning electron microscope (SEM) on cleaved samples. Thefilm hardness was investigated by nanoindentation (UMIS-2000,Fischer-Cripps Laboratories). Time of flight-energy elastic recoil de-tection analysis (ToF-E ERDA) was used to determine the elementalcomposition in the films using 32 MeV iodine ions. The experimentaldetails of the ToF-E ERDA can be found elsewhere [21].
Fig. 2. A typical superimposed DC+HiPP discharge pulse used in the present study. Theapplied DC voltage is about −320 V as can be seen before the onset of the HiPP (att=0 μs).
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3. Results and discussion
3.1. Plasma characteristics
The novel PECVD process was run in both DC as well as HiPP+DCmode varying the average power between 50 and 200 W. In Fig. 2 atypical discharge pulse is given when pulsing the hollow cathode. Ingeneral, the peak current and power increase from about 0.5 A and150 W in the DC case to 12 A and 1100 W in the HiPP+DC mode,although the time-average power is kept on a DC level. The instanta-neous increase of applied power to the cathode during the dischargepulse (leading to an increased amount of charge carriers) is known togenerate a denser plasma [22] and thereby an increased probabilityfor the neutral species (Ar, CxHy, and Cu) to undergo electron impactionization. By using optical emission spectroscopy it was possible toinvestigate in more detail the changes in the degree of ionization be-tween the different discharge configurations. Fig. 3 shows a compar-ison between optical emission spectra taken from a DC dischargeand a HIPP+DC discharge. In Fig. 3a–b the total average dischargepower was 150 W. In order to increase the effect of the HiPP processthe average power was decreased to 70 W, while still keeping theHiPP contribution to 30 W (see Fig. 3c). Note that the vertical scalein Fig. 3 is arbitrary and independent of each other, which meansthat quantitative comparisons cannot be made. However, there are sig-nificant qualitative differences between the different cases: 1) The pureDC discharge shows large fractions of neutral copper (Cu I) seen in theregions λ=324–327 nm, λ=510–523 nm and λ=578 nm, whereasthe emission from the HiPP+DC discharges also shows singly ionizedcopper (Cu II) in the region λ=404–420 nm [23]. This is particularly
200 400 600 800 1000 1200
a) DC 150 W
Cu I
Ar I
Ar I
Cu I
Ar I
Ar I
Ar II
Cu II + C II
Ar I
Ar ICu I
Ar IICu II + C II
C II
Cu I
Cu I
Cu I
b) HiPP+DC 150 W
c) HiPP+DC 70 W
Em
issi
on in
tens
ity (
a.u.
)
Wavelength (nm)
Cu I
Cu I
C II
Cu I
Fig. 3. Optical emission spectra taken froma) aDC discharge at an averagepower of 150W,b) a HIPP+DCdischarge at an average power of 150W, and c) a HIPP+DCdischarge at anaverage power of 70W. The different regions indicating neutral and ionized species aregiven.
striking in Fig. 3c, where the HiPP contribution to the overall dischargeis considerably higher. 2) Emission of singly ionized argon (Ar II) isonly found in the HiPP+DC mode, which can be seen in Fig. 3c atλ=428–437 nm and at λ=459–473. 3) Emission of singly ionized car-bon (C II) is only found in the HiPP+DC mode, which is most clearlyseen at λ=658 nm, but also at λ=426–427 nm (Fig. 3c) [23]. As canbe seen in all cases there is strong emission from neutral argon (Ar I)(λ=696–980 nm) [23]. No clear evidence of neutral carbon (C I)was found, although there are some emission intensity around600–602 nm in Fig. 3b–c, which could be attributed to the presence ofC I. However, this could also be due to Cu II found at λ=600 nm [23].Furthermore the lack of C I spectral lines may be due to that the relativeintensity of the C I emission is at least an order ofmagnitude lower com-pared to Cu I and Ar I [23]. Neutral H (H I) was not detected. Thedetected intensity peak at λ=658 nm (C II) seen in Fig. 3c is howeverfairly close to λ=656 nm (H I), but none of the other strong peaks indi-cating H I, such as λ=389 nm or λ=486 nm, were found.
From the results presented above it can be concluded that theHiPP+DC mode generally provides a more ionized plasma with re-spect to both the sputtered metal atoms as well as the process andprecursor gases. This is likely caused by an increase in plasma densityand possibly also electron temperature [24]. Until firmer experimen-tal investigations on the plasma conditions, such as Langmuir probecharacterization and mass spectrometry, of this type of PECVD pro-cess has been made no quantitative conclusions can be drawn.
3.2. Deposition process characteristics
The deposition rate for various input powers with a constant acet-ylene flow is shown in Fig. 4. It can be seen that the deposition rateincreases linearly with input power up to ~150 W. For higher inputpowers the deposition rate is constant. This suggests that at the pre-cursor flow studied here (3 sccm C2H2), an input power of 150 W isneeded to use the precursors efficiently, regardless of power schemeused. It is interesting to note that for the whole input power rangestudied, a higher deposition rate is obtained when using a combina-tion of HiPP and DC power. This suggests that the pulsed powerscheme leads to a more effective use of the precursors, possibly dueto the higher plasma density in the HiPP+DC plasma. It has previous-ly been noted for amorphous carbon films deposited from acetyleneby ECR-PECVD that the deposition rate increases with ion currentdensity [25]. It is thus likely that the greater amount of ionized spe-cies present in the HiPP+DC mode, as observed in Section 3.1, con-tributes to the approximately 20% increase of the deposition ratecompared to the pure DC mode for the same average input power.
The changes in deposition rate when changing input acetyleneflow rate are shown in Fig. 5. The deposition rate is found to increaselinearly with increasing precursor flow in the precursor flow rangestudied. It can also be noted that the deposition rate is slightly higher
Fig. 4. Deposition rate for different input power using DC power (squares) and HiPP+DC(circles) for a constant C2H2 flow of 3 sccm.
Fig. 5. Deposition rate for different input C2H2flow rates usingDC(squares) andHiPP+DC(circles) for a constant input power of 150W. Fig. 7. Cross section SEM micrograph of a film deposited at three different substrate bias
values (−16 V, -100 V, and−200 V) to study the effect of the substrate bias on themicro-structure and deposition rate of the film. Deposition was done for 5 minutes for each sub-strate bias with an acetylene flow of 6 sccm and an input power of 150W HiPP+DC.
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when using HiPP+DC power compared to using DC power. This is afurther indication that the pulsed power leads to increased plasma re-activity, increased cracking of the precursor gases, and ultimately amore efficient PECVD process.
3.3. Film characteristics
The deposited amorphous, copper containing carbonfilms all appeardense in cross sectional SEM (Fig. 6). From ToF-E ERDA, it was foundthat the films contained 30–50 at% copper, emanating from sputteringof copper from the hollow cathode. The amount of copper in the filmswas found to decrease with higher deposition rate, achieved by higherinput power (Fig. 4). Also by using HiPP+DC, less copper was incorpo-rated in the films compared to when using pure DC. This is somewhatsurprising given the higher ionization of the plasma when HiPP+DCwas used. It could be speculated that the higher ionization of the plasmagives an increased ion bombardment of the film which might lead to apreferential re-sputtering of the copper atoms. Another possibility isback-attraction of copper ions to the negatively charged cathode [26],hence decreasing the metal flux to the substrate. Naturally, the amountof sputtered atoms from the cathode is highly dependent of the sputteryield of the cathode material used and a cathode material suitable forthe deposited thin film should be used. However, the sputtering ofatoms from the cathode suggests that the presented PECVD methodcould be used for processes that use atoms both sputtered from asolid material and released by chemical reactions from gases. Such pro-cesses can be regarded as hybrids of CVD and PVD and could be benefi-cial for depositing films containing metals for which the standard CVD
Fig. 6. Cross section SEM micrograph of an XRD amorphous carbon film deposited at20 μm/h with 3 sccm C2H2 and input power of 150 W HiPP+DC.
precursors are undesirable to use due to e.g. high price, high toxicityor that the precursor molecule contains atoms detrimental to the film.From the ERDAmeasurements it was also concluded that the hydrogencontent in the films was 4–5 at% which is significantly lower than re-cently reported hydrogen content of 20–35 at% for amorphous carbonfilms, free from copper, deposited by PECVD [27]. The reason for hydro-gen elimination is likely to be a combination of large energy fluxes tothe substrate, which will raise the substrate temperature as well asion bombardment of the growing film resulting in preferential removalof hydrogen [1,28]. The hardness of the films was in the order of 5 GPa,which is expected, since the copper content is fairly high in all thedeposited samples and thus reducing the hardness [29]. Further optimi-zation of the coating properties will be the aim of a future study.
The applied substrate AC bias was observed to have a significantimpact on the film microstructure as seen in Fig. 7, where differentsubstrate bias has been used during film deposition. The depositionwas first done for 5 minutes with −200 V AC bias and resulted in1.2 μm film, corresponding to a deposition rate of 14 μm/h, with a fairlydense, non-columnar microstructure. The substrate bias was then de-creased to −100 V AC with continued deposition for an additional5 minutes resulting in 1.6 μm film, corresponding to 19 μm/h, with aless dense microstructure. Finally, the substrate bias was adjusted to−16 V AC which after 5 minutes deposition yielded 2.3 μm film, whichgives a deposition rate of 27 μm/h, with a highly porous, columnarmicrostructure.
The initially high negative bias voltage used, leads to an increasedkinetic energy of the charged particles bombarding the film surface,which is known to generate a denser, less columnar structure, wherere-nucleation commonly takes place [30]. As the substrate bias voltageis reduced the growth mode is shifted towards an under-dense colum-nar morphology due to limited surface diffusion. Worth noting is thatthe best results regarding hardness stems from deposition processesusing −200 V.
4. Concluding remarks
A novel PECVD method based on a hollow cathode discharge hasbeen presented and used for high rate deposition of amorphous, coppercontaining carbon thin films. The study presented shows that by apply-ing a combination of high power pulses (HiPP) and DC power to thecathode, a plasma-based process that uses the precursor gases more ef-ficiently is obtained. It is likely an effect of an increased plasma densityresulting in an increased degree of ionization of the sputtered metalatoms as well as the process and precursor gases compared to a pureDC discharge. This PECVD method further has the ability to supply
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atoms for the films also by sputtering from the hollow cathode and it isthereby potentially interesting for processes using metals for whichstandard CVD precursors are undesirable e.g. due to their toxic or ex-pensive nature.
Acknowledgments
Financial support from the Swedish Innovation Agency (VINNOVA)and Ångpanneföreningens forskningsstiftelse (ÅForsk) is gratefully ac-knowledged. Ionautics AB is gratefully acknowledged for providing aHiP3 power supply.
References
[1] W. Hess, D.B. Graves, in:M.L. Hitchman, K.F. Jensen (Eds.), Chemical Vapor Deposition:Principles and Applications, Academic Press Ltd., San Diego, 1993, p. 385.
[2] L. Martinu, O. Zabeida, J.E. Klemberg-Sapieha, in: P.M. Martin (Ed.), Handbook ofDeposition Technologies for Films and Coatings, third ed, Elsevier, Amsterdam,2010, p. 392.
[3] S.E. Alexandrov,M.L. Hitchman, in: A.C. Jones,M.L. Hitchamn (Eds.), Chemical VapourDeposition: Precursors, Processes and Applications, Royal Society of Chemistry,Cambridge, 2009, p. 494.
[4] A.D. White, J. Appl. Phys. 30 (1959) 711.[5] H. Morgner, M. Neumann, S. Straach, M. Krug, Surf. Coat. Technol. 108–109
(1998) 513.[6] D. Söderström, Doctoral Thesis: Modelling and Applications of the Hollow Cathode
Plasma, Uppsala University, Uppsala, Sweden, 2008.[7] H. Bolt, A. Buuron, F. Koch, M. Noethe, Surf. Coat. Technol. 108–109 (1998) 520.[8] A. Hellmich, T. Jung, A. Kielhorn, M. Rissland, Surf. Coat. Technol. 98 (1998) 1541.[9] D. Lusk, T. Casserly, M. Gupta, K. Boinapally, Y. Cao, R. Ramamurti, P. Desai, Plasma
Process. Polym. 6 (2009) S429.
[10] J. Ni, W. Wu, X. Jin, X. Yang, Z. Chen, Y. Tang, Thin Solid Films 516 (2008) 7422.[11] U. Helmersson, M. Lattemann, J. Bohlmark, A.P. Ehiasarian, J.T. Gudmundsson,
Thin Solid Films 513 (2006) 1.[12] D. Lundin, K. Sarakinos, J. Mater. Res. 27 (2012) 780.[13] K. Jiang, K. Sarakinos, S. Konstantinidinis, J.M. Schneider, J. Phys. D 43 (2010)
325202.[14] R. Bandorf, D. Loch, G. Bräuer, 54th Annual Technical Conference Proceedings of
the Society of Vacuum Coaters, 2011, p. 155, (Chicago, IL, April 16–21).[15] I. Konyashin, J. Bill, F. Aldinger, Chem. Vap. Depos. 3 (1997) 239.[16] J. Robertsson, Phys. Status Solidi A 205 (2008) 2233.[17] J. Robertson, Mater. Sci. Eng. R 37 (2002) 129.[18] C.C. Chen, F.C.N. Hong, Appl. Surf. Sci. 242 (2005) 261.[19] R.M. Balestra, A.M.G. Castro, M. Evaristo, A. Escuderio, P. Mutafov, T. Polcar, A.
Cavaliero, Surf. Coat. Technol. 205 (2011) S79.[20] Y.H. Chan, C.F. Huang, K.L. Ou, P.W. Peng, Surf. Coat. Technol. 206 (2011) 1037.[21] M. Junaid, D. Lundin, J. Palisaitis, C.-L. Hsiao, V. Darakchieva, J. Jensen, P.O.A. Persson,
P. Sandstrom, W.-J. Lai, L.-C. Chen, K.-H. Chen, U. Helmersson, L. Hultman, J. Birch,J. Appl. Phys. 110 (2011) 123519.
[22] J.T. Gudmundsson, Vacuum 84 (2010) 1360.[23] NIST Atomic Spectra Database (ver. 4.1.0) [Online]. Available: http://physics.nist.gov/
asd [2012, January 20].[24] J.A. Hopwood, in: J.A. Hopwood (Ed.), Thin Films: Ionized Physical Vapor Deposition,
Academic Press, San Diego, 2000, p. 191.[25] N.A. Morrison, S.E. Rodil, A.C. Ferrari, J. Robertson, W.I. Milne, Thin Solid Films 337
(1999) 71.[26] D.J. Christie, J. Vac. Sci. Technol., A 23 (2005) 330.[27] S. Peter, M. Günther, D. Hauschild, D. Grambole, F. Richter, Vacuum 85 (2010)
510.[28] W.A.P. Claassen, W.G.J.N. Valkenburg, M.F.C. Willemsen, W.M.v.d. Wijgert, J. Elec-
trochem. Soc. 132 (1985) 893.[29] Q. Wei, R.J. Narayan, A.K. Sharma, J. Sankar, J. Narayan, J. Vac. Sci. Technol., A 17
(1999) 3406.[30] K. Sarakinos, J. Alami, S. Konstantinidis, Surf. Coat. Technol. 204 (2010) 1661.